1. Field of the Invention
The present invention relates to electrical trigger circuits for gaseous discharge flashtubes, and more particularly, to trigger circuits for flashtubes that must be triggered reliably at low anode power supply voltages.
2. Description of the Prior Art
As illustrated in FIGS. 2 and 2B, prior art flashtube trigger circuits generally trigger flashtubes by applying a high voltage pulse to the flashtube gas by either a direct series triggering method of injecting a high voltage pulse in series with the flashtube anode or cathode circuit or by a capacitively coupled external trigger method. FIG. 2A depicts the flashtube anode voltage waveform during the trigger event of FIG. 2.
The minimum cathode to anode operating voltage of a flashtube is determined by lamp element geometry, gas fill pressure and lamp construction materials. Flashtube discharge is initiated by the application of a high voltage trigger pulse greater than the static breakdown voltage of the tube, generally ranging between 2000 to 20,000 volts. The difference between the trigger voltage and the lamp operating voltage must be sufficient to avoid spontaneous triggering. A ratio of 10:1 minimum is typically used to prevent spontaneous triggering.
The direct series triggering method utilizes a large trigger transformer with a secondary winding connected in series with either the lamp cathode or anode to inject a high voltage pulse when a semiconductor or mechanical switch is closed to initiate a trigger event. Closure of the trigger switch discharges a small trigger capacitor through the trigger transformer primary winding which induces a damped high voltage oscillation in the secondary winding. Direct series trigger components are large and costly because they must carry the full flashtube electrode current. The maximum anode voltage applied to the flashtube during the trigger event is the sum of the voltage of the power supply energy storage capacitor and the trigger transformer voltage.
The capacitivity coupled external triggering method is used with flashtubes that have an external trigger electrode fastened to the flashtube which extends over the entire arc length of the tube.
The external trigger electrode forms a capacitance of approximately 10 pf against the cathode and anode of the lamp. As a result, a small pulse transformer with a transformation ratio of 1:20 to 1:100 is used to generate a high voltage pulse when a semiconductor or mechanical trigger switch is closed to start a trigger event. The resulting discharge of the small trigger capacitor into the trigger transformer primary winding produces a damped high voltage oscillation in the secondary winding. The maximum anode voltage applied to the flashtube during the trigger event by this circuit equals the power supply energy storage capacitor voltage.
Other prior art variations of the capacitive external triggering method provide an increase in flashtube cathode to anode voltage during a trigger event by using an auxiliary anode voltage supply having an output voltage higher than the power supply energy storage capacitor voltage to assist lamp triggering. U.S. Pat. No. 4,900,990 teaches capacitive triggering with an external anode boost voltage source. Page 7 of the 1992 Heimann Optoelectronics Flashtube Guide teaches the use of a voltage doubling circuit that requires four electrical connections to the lamp assembly and a diode and small capacitor to increase the apparent anode voltage on the lamp during the trigger event.
As illustrated in FIG. 3, the prior art voltage doubler taught by Heimann requires four electrical connections to the remote lamp assembly and therefore will not work with the large number of three wire flashtube assemblies currently in use. The FIG. 3A timing diagram graphically represents the anode voltage change during a trigger event relating to the circuit illustrated in the FIG. 3 electrical schematic diagram.